The thesan-zoom project: The Hidden Neighbours of O i Absorbers during Reionization

Abstract Metal absorbers represent a powerful probe of galaxy feedback and reionization, as highlighted by both observational and theoretical results showing an increased abundance of low-ionised metal species at higher redshifts. The origin of such absorbers is currently largely unknown because of the low number of galaxy counterparts detected, suggesting that they might be surrounded by low-mass faint sources below the current detection threshold. We use the thesan-zoom radiation hydrodynamic simulations to investigate the connection between properties of neutral oxygen (O i) absorbers and galaxies across redshift z = 5 − 8. We find that the virialised gas in haloes becomes progressively ionised with cosmic time, leading to a decrease of ≈0.2 in the covering fraction of neutral oxygen, while the total oxygen covering fraction remains constant. Comparing the O i line density obtained from our covering fractions with the trend suggested by blind quasar observations, we determine that the observable absorbers ($N_{O\, {\small I}} \gtrsim 10^{13}\, \textrm{cm}^{-2}$) are not confined to haloes: at z ≥ 5 the majority (≳ 60 %) arise beyond Rvir, consistent with recent JWST results. Close to O i absorbers, low-mass galaxies ($M_\star \le 10^8\, \rm {M}_\odot$) are more commonly found, while, depending on the simulated environment, we do not exclude the possibility of nearby more massive star-forming sources (≥5 M⊙ yr−1) similar to those suggested by the latest ALMA observations. These results establish O i absorbers as sensitive tracers of the evolving ionisation structure around faint galaxies to be probed by forthcoming deep spectroscopic surveys.